Free Astronomy Magazine November-December 2022

19 NOVEMBER-DECEMBER 2022 ASTRO PUBLISHING E SO’s Very Large Telescope (VLT) has imaged the result of a spec- tacular cosmic collision — the galaxy NGC 7727. This giant was born from the merger of two galax- ies, an event that started around a billion years ago. At its centre lies the closest pair of supermassive black holes ever found, two objects that are destined to coalesce into an even more massive black hole. Just as you may bump into someone on a busy street, galaxies too can bump into each other. But while galactic interactions are much more violent than a bump on a busy street, individual stars don’t gener- ally collide since, compared to their sizes, the distances between them are very large. Rather, the galaxies dance around each other, with grav- ity creating tidal forces that dramat- ically change the look of the two dance partners. ‘Tails’ of stars, gas and dust are spun around the galax- ies as they eventually form a new, merged galaxy, resulting in the dis- ordered and beautifully asymmetri- cal shape that we see in NGC 7727. The consequences of this cosmic bump are spectacularly evident in this image of the galaxy, taken with the FOcal Reducer and low disper- sion Spectrograph 2 (FORS2) instru- ment at ESO’s VLT. While the galaxy was previously captured by another ESO telescope, this new image shows more intricate details both within the main body of the galaxy and in the faint tails around it. In this ESO VLT image we see the tangled trails created as the two galaxies merged, stripping stars and dust from each other to create the spectacular long arms embracing NGC 7727. Parts of these arms are dotted with stars, which appear as bright blue-purplish spots in this image. Also visible in this image are two bright points at the centre of the galaxy, another telltale sign of its dramatic past. The core of NGC 7727 still consists of the original two galactic cores, each hosting a super- massive black hole. Located about 89 million light-years away from Earth, in the constellation of Aquar- ius, this is the closest pair of super- massive black holes to us. The black holes in NGC 7727 are ob- served to be just 1600 light-years apart in the sky and are expected to merge within 250 million years, the blink of an eye in astronomical time. When the black holes merge they will create an even more massive black hole. The search for similarly hidden supermassive black hole pairs is expected to make a great leap forward with ESO’s upcoming Extremely Large Telescope (ELT), set to start operating later this decade in Chile’s Atacama Desert. With the ELT, we can expect many more of these discoveries at the centres of galaxies. Our home galaxy, which also sports a supermassive black hole at its cen- tre, is on a path to merge with our closest large neighbour, the An- dromeda Galaxy, billions of years from now. Perhaps the resulting galaxy will look something similar to the cosmic dance we see in NGC 7727, so this image could be giving us a glimpse into the future. ! C lose-up view of the two bright galactic nuclei, each housing a supermassive black hole, in NGC 7727. Each nu- cleus consists of a dense group of stars with a supermassive black hole at its centre. The two black holes are on a colli- sion course and form the clos- est pair of supermassive black holes found to date. It is also the pair with the smallest sep- aration between two super- massive black holes − observed just 1600 light-years apart in the sky. [ESO/Voggel et al.]